1. Field of Invention
The present invention relates to a method for dynamically adjusting the spectral content of a digital audio signal wherein significant processing is performed to modify a signal's harmonic content
2. Background of the Invention
Much audio is stored, distributed and processed in the digital domain. Regardless of this fact, the audio must ultimately be converted back to analog in order to be used. Many audio purists resist the digitization of audio, preferring pure analog sources such as LP recordings, which originate from analog master tapes. This is because of inherent defects in what are termed “lossy compression” and “lossless compression” in audio data compression. In both lossy and lossless compression, information redundancy is reduced, using methods such as coding, pattern recognition and linear prediction to reduce the amount of information used to describe the data. The idea behind lossy audio compression was to use psychoacoustics to recognize that not all data in an audio stream can be perceived by the human auditory system. Most lossy compression reduces perceptual redundancy by first identifying sounds which are considered perceptually insignificant. Typical examples include high frequencies, or sounds that occur at the same time as other louder sound, which are coded with decreased accuracy or not coded at all.
However, reducing perceptual redundancy often does not achieve sufficient compression for a particular application and requires further lossy compression with a difference in quality that is more readily perceived by the user. While the data reduction is again guided by some model of how important the sound is as perceived by the human ear, with the goal of efficiency and optimized quality for the target data rate, the use of lossy compression may result in a perceived reduction of the audio quality that ranges from none to severe.
Currently, data removed during lossy compression cannot be recovered by decompression. Additionally, audio quality is affected when a file is decompressed and recompressed (generational losses) which makes lossy compression unsuitable for storing the intermediate results in professional audio engineering applications but makes it very popular with end users (particularly MP3) since a megabyte can store almost a minute's worth of music at adequate quality.
Timbre or tone color is known in psychoacoustics as sound quality or sound color. Timbre has been called “the psychoacoustician's multidimensional wastebasket category” as it can denote many apparently unrelated aspects of sound. McAdams, S., and Bregman, A. “Hearing Musical Streams,” Comput. Music J. It should be pointed out that the addition or restoration of harmonics will have the effect of sharpening the rise of the leading edge of transient signals, this is analogous to edge enhancement in video. It has been observed that the rendering of the leading edge of transient signals is a key element in the perception of tone color or timbre and in the rapid identification of sounds. Thus restoring the harmonics lost to audio compression also serves to restore timbre resulting in a higher quality listening experience.
While this method is obviously useful for compressed digital audio signals, it is also useful to enhance non-compressed digital audio signals. This will result in a richer timbre or tone color to the audio signal and an enhanced listening experience.